Microwave heating has demonstrated itself to be a powerful technique for sintering various ceramics, especially through the past decade. Microwave heating may decrease the sintering temperatures and times dramatically, and is economically advantageous due to considerable energy savings. However, one of the major limitations is the volume and/or size of the ceramic products that can be microwave sintered, because of an inhomogenous microwave energy distribution inside the applicator which often results in a non-uniform heating. Considerable research has gone into making microwave sintering technology viable, but batch processing was always a handicap.
Continuous microwave sintering of alumina is a newly developed process. The principle of the continuous microwave sintering furnace is shown in FIG. 1. The microwave applicator is designed to focus the microwave field in the central area as uniformly as possible. A long cylindrical ceramic hollow tube contains the green alumina which is fed into the microwave applicator at a constant feed speed. As the alumina enters the microwave cavity, it is heated and gradually sintered while passing through the microwave zone. The heating rate, sintering time and cooling rate are controlled by the input microwave power, the feeding speed, and the thermal insulation surrounding the heated alumina. The ceramic hollow tube is also rotated during processing for uniform and homogenous heating. As the green alumina passes through the high temperature zone, the particles are sintered entirely. Since the ceramic hollow tube is moved relatively continuously in the axial direction during the processing, there is virtually no limitation to the length of the product that can be processed by this technique. Consequently, it is possible to scale-up the volume of the ceramic products to be microwave sintered by this technique to provide a continuous process.
This disclosure proves the continuous microwave sintering technique for large quantities of commercial alumina grains with properties required for use as an advanced abrasive grain.
The disclosure also is directed to a novel synthesis method for the manufacture of alumina based sol gel abrasive utilizing the newly developed microwave processing. The process offers a faster, energy efficient route to manufacture abrasive alumina grains. Grains prepared by this method exhibited micro hardness above 2200 kg/mm.sup.2, 98% theoretical density, crystalline uniformity and average size less than 0.5 microns with high abrasion properties.
One aspect of this invention relates to improved preparation of alumina abrasives. The alumina particles are used for both coated and bonded abrasives. They are conventionally produced using arc furnace melting technology with either calcined bauxite or calcined alumina as the starting material and at temperatures above 2000.degree. C.
These conventionally prepared alumina abrasives normally have grain sizes above 150 microns. The conventional process for the production of alumina abrasive grains is through fusion using the known arc furnace. The calcined alumina is melted at temperatures above 2000.degree. C. or calcined bauxite is fused at temperatures above 2000.degree. C. in presence of carbon to reduce its impurities such as titania, iron oxide, silica etc.
In both the above cases, the molten material is cooled, crushed, magnetically screened and graded to obtain the various grit sizes required for abrasive grain preparation. Since the process requires melting of the raw material, the process becomes highly energy intensive; worse, the crystal size of the resulting abrasive grain is more than 150 microns in size.
Subsequently, another process was developed through sol gel route yielding abrasive grains with small crystal sizes of about 0.5 microns consuming low energy. In this route, the major steps so far employed include dispersing the alumina oxide (Boehmite) in water to form alumina in sol state, addition of seeds like alpha alumina either by milling using alumina grinding media or direct addition of micronized alpha alumina particles. Then, selected additives such as magnesia, titania, yttria, etc., added; the material is dried, crushed and sintered using conventional sintering furnace. The conventional sintering furnace is typically an electric or oil or gas fired batch kiln with a rotary kiln. The heating rate is about 20.degree. C./minute as the system applies external heating to the material.
As a rule of thumb, the performance of the abrasives with the same hardness, toughness and density improves with decrease in grain size. It is possible to achieve very small grain sizes with high hardness, toughness and density, using the combination of sol gel and microwave processes thereby improving the abrasive characteristics when compared to the conventional process. This process requires lower temperature, around 1400.degree. C.
By the use of the process of the present invention, it is possible to prepare a new variety of alumina abrasive grains at considerably lower energy with small crystal size, high hardness and density. The process of the present invention also involves use of a microwave sintering technique in which higher heating rates are employed to form finer particles than conventional products. In the process of the invention, microwave heat is generated internally within the material instead of originating from external heating sources and is a function of the material being processed. It is seen that as the temperature increases above a certain point, the dielectric loss begins to increase rapidly and the material begins to absorb microwaves more efficiently. This also raises the temperature very rapidly. In many cases the heating rates are as high as 300.degree. C./minute. Both batch and continuous processing systems can be employed.
The raw material used in the process of the present invention is boehmite dispersed in water with a concentration in the range of 15 to 30 weight percentage. The pH is adjusted between 2 to 4 by controlled addition of an acid such as nitric acid and peptized. The additives such as iron, silicon, titanium, magnesium, yttrium, neodymium, lanthanum etc., are added as their hydroxides, oxides or nitrates. The amount of additives may vary from 0.1 to 7 weight percentage when considered as their oxides. Seeding involves sub micron alpha alumina seeds in the amount of up to 1.5% by weight. Control drying chemical additives may be added in the range of 0.1 to 1.0% to ensure uniform heating of the mixture and to aid pore free drying. The gel prepared is dried at the temperatures of 600.degree. to 800.degree. C. The dried gel is crushed to the required size giving allowance for the shrinkage during sintering. The shape of the grains is modified to suit the end use by selection of a suitable crushing system.
The graded gel is then calcined in the temperature range of 300.degree. to 900.degree. C. for about 30 minutes to drive away the volatiles. The calcined grains are then sintered using microwave technology at temperatures less than 1500.degree. C.
Another object of the present invention is to provide an improved process for the preparation of alumina abrasive grains having low crystal sizes, high hardness and toughness and density overcoming the drawbacks of the hitherto known processes.
Yet another object of the present invention is to produce an improved process for the preparation of alumina abrasive grains having Micro Vickers hardness above 2100 kg/sq. mm, 90% theoretical density with crystalline uniformity and average crystal size less than 0-6 microns.